As the waters fail from the sea. The allusion seems to be to the actual desiccation of seas and rivers. Job, apparently, had known instances of both. A formation of new land in the place, of sea is always going on at the head of the Persian Gulf, through the deposits of the Tigris and Euphrates; and this formation was very rapid in ancient times, when the head of the gulf was narrower.
The desiccation of river-courses is common in Mesopotamia, where arms thrown out by the Tigris and Euphrates get blocked, and then silted up. And the flood decayeth and drieth up; rather, and the river decayeth' etc.
(see the comment on the preceding clause).